"Meat Plow" wrote in message
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On Thu, 11 Oct 2007 22:16:55 -0500, Don Young wrote:


"Meat Plow" wrote in message
...
On Thu, 11 Oct 2007 18:14:42 -0500, Moe Jones wrote:

Meat Plow wrote:
On Thu, 11 Oct 2007 05:21:58 +0000, PaPaPeng wrote:

My gas furnace has a thermocouple problem. Stronmg winds blew out
the pilot light. I can relight the pilot light, but no heat.

Thermocouple is ok if it holds the pilot valve open. Need to verify
that there is a call for heat via the thermostat and the low voltage
supply is available. Check for 18 - 24 volts AC at the gas valve. If
so then the vavle is stuck or the coil is open. If not then check
your low voltage transformer and wiring. BTW the thermocouple should
put out maybe a tenth of a volt or so.


You are right and I total agree with you. He may have a defective gas
valve
and if he does did the valve just wear out or is there a short in the
system, that took the valve out?

Well he posted again and said he modified the way the new slim
thermocouple mounted and now all is well. I suppose if the gas valve was
stuck shut that all the vibration from drilling etc.. could have unstuck
it. It's been years and I don't recall if I've run into a situation
where
a thermocouple could have just enough output to hold the pilot open but
not enough for the safety on the main valve or even if if the older
valves
were designed in a way that this could be possible. I have my doubts
that
this is the end of his problems. As far as a short taking out the gas
valve on a 20 year old system, that's not likely to happen as a short
would remove the voltage path to the gas valve and possible take
something
else out instead.
Some of the old gas valves supplied gas to the pilot independent of the
thermocouple. If the pilot went out or the thermocouple went bad, the
pilot
gas was not cut off. Some systems even had a small separate valve which
supplied gas to the pilot, totally independent of the automatic main gas
valve.

Don Young

Seems very hazardous to allow gas to the pilot if there isn't a flame to
burn it doesn't it? I would hope this wasn't the case in valves
manufactured within the past 50 years or so during which heating migrated
from coal to gas.
Yes, there was a hazard which is why that type is no longer used. I think
the idea was that such a small amount of gas was not likely to be a problem
as long as the venting was good. I do not know when they went out of use but
it may have been when LPG became common since LPG is heaver than air and
does not dissipate as well. Cookstove pilots used to be that way too and I
do not know if any are still.

Don Young